Conventional thermal cracking or pyrolysis of naphtha into olefins have been carried out in a naphtha-cracking system wherein all of the charged feed stock is evaporated in a preheater tube provided in a convection section of a thermal cracking furnace, thermally cracked in a reaction tube provided in a radiant section of the thermal cracking furnace, and then cooled in a quenching heat exchanger. Such a conventional cracking system had a construction as illustrated in FIG. 3. As shown in FIG. 3, a cracking system 60 comprises a thermal cracking furnace 12, a quenching heat exchanger 14, and numerous lines. The thermal cracking furnace 12 is divided into a convection section 18 of the thermal cracking furnace and a radiant section 20 of the thermal cracking furnace. In the convection section 18 of the thermal cracking furnace, a feed stock a' such as naphtha is introduced through a feed stock-supplying line 34 provided outside the furnace into a first preheater 22 where the feed stock a' is preheated to produce a preheated feed stock b'. The preheated feed stock b' is introduced through a connecting line 62 to a second preheater 26 where the feed stock b' is additionally preheated to produce a fully preheated feed stock i'. The preheated feed stock b' is evaporated prior to its introduction into the second preheater 26 by admixing superheated dilution steam c supplied through a connecting line 64. The superheated dilution steam c is produced in a dilution steam superheater tube 28 wherein steam introduced through a dilution steam-supplying line 44 from outside the furnace is superheated.
The fully preheated feed stock i' preheated in the second preheater 26 is passed through a connecting line 46 to a thermal cracking reactor 30, where it is thermally cracked into reaction products j. The reaction products j are passed through a connecting line 48 to a quenching heat exchanger 14, where they are cooled to produce cooled products k. The cooled products k are passed through a product discharge line 50 to further processing.
The conventional cracking system 60 as described above has been effective for cracking high-quality feed stocks such as naphtha.
However, cracking of low-quality feed stocks such as HNGL (heavy natural gas liquid, an associated oil occurring in small quantity with production of gas from gas fields) is recently required in addition to naphtha which has been conventionally employed for the cracking purpose.